The study aims to investigate the influence of the online professional development (PD) program focused on precollege engineering education on science teachers' teaching engineering self-efficacy. A nested mixed research design was employed to conduct the study, in which the basic qualitative design was integrated into a weak experimental design and data collection was interrelated. The quantitative dimension of the study utilized the engineering teaching self-efficacy scale, while the qualitative dimension utilized the teachers' reflections as a data collection instrument. Fourteen science educators were selected through purposive random sampling in a province in the Black Sea region of Türkiye. The study's results suggested that the self-efficacy of science teachers in teaching engineering was significantly enhanced by the PD program, which focused on engineering education. The qualitative results were consistent with the quantitative results. Considering the study's results, practitioners and researchers were provided with suggestions for future research in the field of teacher education or PD programs.